This invention relates to an improved anticoagulant composition for use with mammalian whole blood.
Hematological tests in human and veterinary medicine are employed for a host of diagnostic aids, and numerous blood anticoagulant mixtures have been widely employed to permit testing and measurement of whole blood properties without clotting. Citrate anticoagulants, the material of choice for blood banks and transfusion purposes, have never gained acceptance for hematological testing purposes. Oxalates are still used for hematological purposes but must be used rapidly and in fairly large quantities in order to avoid drastic influences on cellular morphology and distortion of hematocrit values. The coumadins, of great value as in vivo anticoagulants, are not suitable for in vitro applications due to the anticoagulating mechanism characteristic of these compounds. Heparin, although relatively expensive and unstable, is useful in blood gas analyses and ion concentration measurement due to its nonionic structure. However, although to a lesser extent than the oxalates, heparin also adversely affects the morphology and in particular the staining properties of white blood cells.
EDTA (ethylenediaminetetracetate) has today become the anticoagulant of choice for most routine hematology procedures. While the literature and product specifications are often unclear in using the designation EDTA as a generic term for the free acid and its various simple or complex (mixed) salts, normally with an alkali metal or a divalent alkaline earth metal cation, the potassium and sodium salts Na.sub.2 EDTA and K.sub.3 EDTA are at present most widely employed in this country.
For purposes of collecting blood samples to be used in hematological testing, anticoagulants are often predispensed in evacuated collection tubes designed to draw a specified volume of a whole blood sample. In practice, the use of such tubes does not always entail collecting a full sample because of premature withdrawal, weakened vacuum or a number of other practical problems. Since very small quantities of blood are required for most hematology procedures, the sample will normally be adequate in terms of required volume and sent to the laboratory for testing anyway. The net effect of such occurrences results in a sample containing an excess concentration of anticoagulant over that which would be obtained by collecting a "full draft" sample. As shown in more detail hereinafter, variations in concentration of anticoagulants now employed can distort hematological test results. While perhaps not significant for many purposes, the distortion in lowering hematocrit values can be a serious matter for a patient or animal suffering dehydration, serious burns or the like and/or being prepared for treatment with an artificial kidney machine.
Hematocrit values are one of the most commonly employed hematological determinations. While the classic method for determining hematocrit has involved simple centrifugation of blood to which an anticoagulant has been added, electronic cell counters such as the Coulter Counter Model S have recently been employed. In the operation of this electronic cell counter, the blood is diluted 1:62,500 and a measured volume is drawn through an orifice while the electrical conductivity is measured between the two sides of the orifice. Cells have a lower conductivity than the diluting fluid. As they pass through the orifice, a change in conductivity occurs. The magnitude of the change is quantitatively related to the volume of the cells so that the MCV* can be determined. The packed cell volume (PCV) or hematocrit is then calculated as (MCV .times. RBC count)/10. While generally satisfactory, it has been found that discrepancies between centrifugally determined hematocrit values can be masked by such electronic cell counters. Since the presently employed EDTA salts act to lower hematocrit values this masking presents a real danger to a limited number of patients. Accordingly, there is a need for a blood anticoagulant composition which does not suffer the above mentioned deficiencies of prior art compositions. FNT * Mean corpuscular volume